The present invention relates to a tool holder for a tool-bit provided with a shaft. Such tool holders are normally arranged in a machine of one type or another, for example a drill or a machine for a mechanical machining process such as milling or grinding. The tool holder generally has an upper end provided with tapping or screw threads enabling it to be screwed into the machine. The upper part is generally conical, but may be cylindrical, and similarly provided with tapping at its upper end. A chuck of some sort is usually fitted at the lower end of the tool holder. The tool-bit with shaft is inserted into the chuck and is secured there by a number of elements which, by means of a hand tool operated by the user, can be caused to exert a clamping action. This can also be achieved by mechanical means. If the recess in the chuck is too large, a sleeve or collect is inserted into the recess, the sleeve having the same outer diameter as the chuck's inner diameter. The inner cross section of the sleeve is adjusted to the cross section of the tool-bit to be used. The retention of a tool-bit is dependent on the force with which a user can secure the tool-bit or which can be achieved by mechanical means. This force is often insufficient and the tool-bit may spin or loosen.